The present invention relates to a cooling arrangement for the cooling air of a jet engine which is operated by means of cryogenically stored fuel, particularly hydrogen. The fuel, after the heat absorption, is fed to the engine in a heat exchanger arranged in line with the fuel feeding pipe. The cooling air which is removed from an air inlet of the engine by way of an air feeding pipe, after the heat loss in the heat exchanger, is fed to the engine for cooling. This type of an arrangement is known from the German Patent Document DE 39 42 022 A.
Because of the high Mach number, the temperatures occurring in a jet engine for high-speed aircraft are extremely high. This is particularly true for hypersonic aircraft. As a result, there is a high amount of heat that must be carried away in order to maintain the material temperature of the components in the hot area of the engine within the permissible technical limits. There is the additional problem in this case that the air temperatures in the air inlet of a hypersonic engine in the case of Mach numbers in the range of from 5 to 7, reach 1,500 K to 2,300 K. A cooling of the hot engine components, for example, in the area of the combustion chamber, the turbine and parts connected behind them, such as afterburner elements, by such heated air is particularly difficult. The heated air is therefore fed to a cooling arrangement where it is cooled intermediately in a heat exchanger while the cooling capacity of the fuel is used which is carried along in a liquid state but is burnt in the gaseous state. Hydrogen is particularly suitable for this purpose. The permissible heating of the hydrogen is the result of the fact that the exchange elements of the heat exchanger which, at the same time, are used to separate the hydrogen from the air, in the case of a metallic construction, must be held below maximal temperatures of from 1,000 K to 1,100 K. Locally, exchange elements are stressed by temperatures in the range of the air inlet temperature.
Because of the low inlet temperature of the fuel, there is the danger, however, that the temperature of the cooled air may locally fall below 273 K so that the water contained in the air will precipitate on the walls of the heat exchanger or on the exchange matrix as ice. This clogs the heat exchanger on the air side. This danger becomes acute specifically in the case of lower Mach numbers, thus also in the case of supersonic and subsonic aircraft with cryogenically stored fuel since here the heating of the air is lower. These problems apply particularly to the arrangement that has become known on the basis of the German Patent Document DE 39 42 022 A1. This danger also exists for the heat exchanger of a jet engine according to the U.S. Patent Document 3,733,826 which is arranged for the cooling of the combustion air between the air inlet and the compressor or for the heating of the fuel.
There is therefore needed a cooling arrangement of the above-mentioned type which avoids the mentioned problems and permits a high degree of exchange while the constructional expenditures are as low as possible. In addition, a compact and operationally reliable heat exchanger is to be provided which can be easily integrated into a cooling arrangement of this type.
According to the invention, these needs are met by a cooling arrangement for the cooling air of a jet engine which is operated by means of cryogenically stored fuel, particularly hydrogen. The fuel, after the heat absorption, is fed to the engine in a heat exchanger arranged in line with the fuel feeding pipe. The cooling air which is removed from an air inlet of the engine by way of an air feeding pipe, after the heat loss in the heat exchanger, is fed to the engine for the cooling. A partial flow of the evaporated fuel flowing out of the heat exchanger, delivered by a controllable recirculating blower, by way of a fuel mixing device, is fed to the fuel before it flows through the matrix of the heat exchanger. A cooling air compressor can be arranged in the cooling air pipe between the heat exchanger and the engine. A partial flow of the gas downstream of the matrix is delivered by means of a controllable gas blower arranged in the hollow cylinder.
The arrangement according to the invention has the advantage that, according to the required air outlet temperature, the fuel temperature at the inlet as well as at the outlet of the heat exchanger can be adjusted by a corresponding control of a recirculating blower. Icing of the heat exchanger can therefore be avoided largely independently of the momentary fuel consumption of the engine which significantly influences the cooling performance of the heat exchanger. By the recirculating of a partial flow of the fuel, the temperature differences are decreased mainly in the area of the exchange matrix and the stress to the material is therefore reduced. In the case of the countercurrent heat exchanger, this is particularly true in the case of the air outlet area of the matrix. This results in an operational safety that is absolutely necessary for aviation, particularly in view of the use of hydrogen as the working medium of the heat exchanger. An engine-caused limiting of the flight envelope can be reduced in that the feeding of the partial flow T takes place while the flight condition values are taken into account.
Another measure for avoiding extreme stress to material because of temperature contrasts, particularly in the area of the air inlet side of the matrix, consists of the construction of the arrangement wherein a partial flow of the cooling air is branched off downstream of the matrix and is fed to the cooling air upstream of the matrix by way of a return flow duct. In addition, the required operating values, when a controllable blower arranged in the return flow duct is used, can be controlled largely variably in cooperation with the recirculating blower.
Because of the fact that, in the area of the fuel outlet of the matrix, fuel already heated flows through the matrix, this matrix is subjected to higher temperature-caused stress. In order to be able to avoid undesirably high temperatures, it is provided to feed to the cooling air in the area of the fuel outlet of the matrix, at least a fraction of the partial flow L in a concentrated manner.
In order to be able to also supply turbine stages with cooling air, it is necessary to compress the cooling air to the pressure level inside the turbines. For this reason, the arrangement of a cooling air compressor downstream of the heat exchanger is suggested which results in a lower required compressor power than in the case of the compression of uncooled air.
A space-saving construction of a heat exchanger is obtained by integrating a drum-shaped matrix into the cylindrical pressure housing. The cylindrical construction of the heat exchanger ensures a reliable pressure tightness of the enclosing pressure housing and therefore meets the high safety requirements when used in an aircraft.
In order to avoid thermal problems on the heat exchanger also in the case of extremely high gas entry temperatures, which occur particularly in hypersonic flight, in a consequent space-saving construction, a partial flow L, by way of the return flow duct in the interior of the hollow cylinder, can be admixed to the uncooled gas flow upstream of the matrix. By means of this construction, it is possible to lower the gas entry temperature in front of the matrix to such an extent that the behavior of the material can be controlled at the operating temperature. Furthermore, a homogeneous mixing of the gas flow with the partial flow can be achieved. This is important for the operating behavior.
In order to avoid local excess temperatures on the matrix, it is recommended in this case to arrange the gas mixing device or at least an additional gas mixing device in the area of the cooling medium outlet of the matrix for the concentrated admixing in this area.
In an expedient combination of the invention with the characteristics that the gas mixing device or an additional gas mixing device is arranged in the area of the cooling medium outlet of the matrix for the concentrated admixing of gases of the partial flow. Even if the cooling requirement fluctuates considerably together with the flight Mach number, the heat exchanger components may be kept in a narrow temperature range which benefits the stability as well as the operational reliability.
Additional advantageous constructions with respect to the heat exchanger with a drum-shaped matrix are also provided.
For achieving a high degree of exchange, while the utilization of space of a heat exchanger with a cylindrical pressure housing is acceptable, a construction having the characteristics that at least two drum-shaped matrices are arranged behind one another in the ring duct and are connected in series by way of central pipes, forming a cross countercurrent matrix. This construction was found to be advantageous. Preferably, it is also provided to equip the matrix with hairpin-shaped special section tube bows which project into the flow direction and which are connected to two central pipes which supply and remove cooling medium. Expansions of the special section tube bows can therefore develop in the longitudinal direction of the flow without any tension.
Additional advantageous developments of the heat exchanger with respect to low pressure losses in the gas and an optimal degree of exchange are also provided.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.